A bicycle is normally provided with two gearshifts, a front gearshift associated with the crankset and a rear gearshift associated with the sprocket set. In both cases, the gearshift comprises a derailleur that engages the transmission chain moving it on toothed wheel with different diameters and number of teeth, so as to obtain different transmission ratios; the derailleur, be it that of the rear gearshift or that of the front gearshift, is moved in one direction by a traction action applied by an inextensible cable that is usually sheathed (normally known as a Bowden cable), in the opposite direction by the elastic return action of a spring provided in the gearshift itself. Normally, the direction in which the displacement is determined by the return spring is that in which the chain passes from a toothed wheel with a larger diameter to a toothed wheel with a smaller diameter, i.e. that of so-called downward gearshifting; vice-versa, the traction action of the control cable takes place in the direction of so-called upward gearshifting, wherein the chain moves from a toothed wheel with a smaller diameter to a toothed wheel with a larger diameter. It should be noted that in a front gearshift downward gearshifting corresponds to a lower transmission ratio, whereas in a rear gearshift it corresponds to a higher transmission ratio.
The movement in the two directions of the control cable of a gearshift is obtained through an actuation device mounted so as to be easily operated by the cyclist, i.e. normally on the handlebars, close to its grips. By convention, the actuation device of the control cable of the front gearshift is adjacent the left grip and vice-versa, the actuation device of the control cable of the rear gearshift is adjacent the right grip.
In the actuation device, the control cable is actuated through winding and unwinding on a rotor element, commonly called a cable-winding bobbin, the rotation of which is controlled by the cyclist with different means according to the type of gearshift. In a typical configuration, the actuation device provides two distinct levers, to control the rotation in the two directions of the bobbin. An indexer (such as that described in the co-pending U.S. application Ser. No. 11/051,908, hereby incorporated by reference) ensures that the bobbin is held still in rotation in a number of predetermined angular positions, corresponding to the different positions of the derailleur required by the different ratios, i.e. on the different sprockets of the gearshift.
The rotatable support of the bobbin in the body or housing of the actuation device is normally obtained thanks to the fact that the bobbin is made with a tubular structure, rotatably mounted on a support pin formed in the housing or anyway integral with it, extending substantially for the entire axial length of the bobbin or for a substantial part thereof. On the outside, the bobbin has a circumferential groove for the winding of the control cable. The pin must withstand relatively high forces, due to the action exerted in the operation by the various elements like the control cable (which is unwound under tension), the indexer and the actuation levers. Clearly, it is necessary to avoid any risk that such forces can cause breaking of the pin; even without breaking, however, an excessive deformation of the pin can lead to imperfect conditions of positioning of the bobbin, resulting in gearshifting difficulties, non-uniform wear of the components, subsequently reducing the service time of the components.
Therefore, there is the problem of making an actuation device in which the cable-winding bobbin is effectively supported, without the risk of breaking or deformation such as to compromise the good operation of the gearshift.